U.S. Pat. No. 3,946,612, incorporated herein by reference, discloses a clinical thermometer comprising a plurality of cavities filled with solid solutions of two thermally responsive materials in each pocket. The melting point of the contents of each pocket varies as a result of the ratio of the two compounds in each pocket. The compositions are compounded to cover the clinical temperature range in 0.2.degree. F. increments. The preferred thermally responsive material comprises a solid solution of ortho-chloronitrobenzene and ortho-bromonitrobenzene (OCNB and OBNB respectively).
The composition of U.S. Pat. No. 3,946,612, is subject to a supercooling phenomenon. Upon melting the composition remains in the liquid state long after the composition has been brought to a temperature below its melting point. For the thermometers of the type contemplated by the prior art his presented problems, and means were devised to cause the compositions to solidify rapidly upon being cooled below their melting point.
U.S. Pat. No. 3,980,581 disclosed compositions useful as nucleating agents in the OCNB/OBNB solid solutions of the prior art. These nucleating agents are salts of metals and include sodium borate, cryolite, sodium acetate and cobalt chloride.
The nucleating agents of the '581 patent though effective were subject to a "poisoning" effect. As the thermally responsive materials were subjected to melting and resolidification cycles the nucleating agent pores eventual became filled with the thermally responsive material. As a result, the nucleating agent became ineffective. This "poisoning" problem was solved by the invention disclosed in U.S. Pat. No. 3,956,153, incorporated herein by reference.
The '153 patent discloses a regenerative nucleating agent comprising a compound which is sparingly soluble in the thermally responsive material. Although a minor amount of material dissolves into the thermally responsive material when it melts, sufficient undissolved material remains to act as a nucleating agent. Because the nucleating agent is sparingly soluble, its surface is continually regenerated, and no poisoning effect is observed.
U.S. Pat. No. 4,397,570 discloses a disposable thermometer similar to that of Hof et al. 4,232,552, incorporated herein by reference, which utilizes a pressure sensitive adhesive containing a nucleating agent to make a thermometer which is reversible and maintains its signal for about 3-5 minutes. The adhesive utilized is the polyisobutylene adhesives of U.S. Pat. No. 4,189,942, incorporated herein by reference. The nucleating agents are generally mineral type compositions such as talc, gibbsite, calcium aluminate, etc.
U.S. Pat. No. 4,248,089 discloses a reversible thermometer wherein the temperature sensitive material comprises a combination of high molecular weight fatty acids. A nucleating agent which both allows the thermometer to be reversible and at the same time hold its signal for a reasonable period of time to be read is included. Suitable nucleating agents include dyes such as Auramine, Acridine Yellow, Red, and Orange; Rhodamine and Sudan Black.
U.S. Pat. No. 4,299,727 to Hof discloses a reversible thermometer utilizing a heat sensitive composition, preferably that of Hof et al. U.S. Pat. No. 4,232,552; a matrix forming amorphous material and a film forming material which is more crystalline than the matrix forming compound. The heat sensitive composition can be a ortho-chloronitrobenzene (OCNB), ortho-bromonitrobenzene (OBNB), 1-thymol, 2-naphthol and naphthaline. The matrix forming amorphous material can comprise microcrystalline waxes and polyisobutylene. Suitable film forming materials include high melting aliphatic alcohols. Optionally, a solubilizing material such as docosanol can be included. The preferred heat sensitive composition is a combination of OCNB and OBNB.
U.S. Pat. No. 4,150,572 discloses a reversible thermometer wherein the signal is maintained for a reasonable period by the addition of a polymer to a thermally sensitive material. The preferred thermally sensitive material is a combination of OCNB and OBNB. The polymer must be soluble in the thermally sensitive material at a level of about 10 to 75 wt. % based on the thermally sensitive material at the melting point of the thermally sensitive material. Suitable polymers include the thermoplastic polymers such as polystyrene, polycrylates and polyvinyl butyral.
Presently, the only commercially available chemical type thermometer utilizing the solid solutions of the '612 patent is a device disclosed and claimed in U.S. Pat. No. 4,232,552, incorporated herein by reference. While the patent discloses compositions utilizing the prior art nucleating agents the device being marketed contains no nucleating agent. That thermometer relies on the supercooling effect to give a signal which is retained for a time sufficient to be read. Since the thermometer is sold as a single use devise, reversal of the signal is not required. Use of the prior art nucleating agents, however, results in a reversal which is too rapid for use where the device is to be read after it is removed from the heat source, the mouth for example. In order for the device of the '552 patent to be reusable it must revert to its original solid state in a reasonable amount of time while at the same time hold a signal to be read for a time sufficient to permit the user to read the temperature shown.